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BBC Micro Elite

Tactics: TACTICS (Part 1 of 7)

Name: TACTICS (Part 1 of 7) [View in context] Type: Subroutine Category: Tactics Summary: Apply tactics: Process missiles, both enemy missiles and our own Deep dive: Program flow of the tactics routine
This section implements missile tactics and is entered at TA18 from the main entry point below, if the current ship is a missile. Specifically: * If E.C.M. is active, destroy the missile * If the missile is hostile towards us, then check how close it is. If it hasn't reached us, jump to part 3 so it can streak towards us, otherwise we've been hit, so process a large amount of damage to our ship * Otherwise see how close the missile is to its target. If it has not yet reached its target, give the target a chance to activate its E.C.M. if it has one, otherwise jump to TA19 with K3 set to the vector from the target to the missile * If it has reached its target and the target is the space station, destroy the missile, potentially damaging us if we are nearby * If it has reached its target and the target is a ship, destroy the missile and the ship, potentially damaging us if we are nearby
.TA34 \ If we get here, the missile is hostile LDA #0 \ Set A to x_hi OR y_hi OR z_hi JSR MAS4 BEQ P%+5 \ If A = 0 then the missile is very close to our ship, \ so skip the following instruction JMP TA21 \ Jump down to part 3 to set up the vectors and skip \ straight to aggressive manoeuvring JSR TA87+3 \ The missile has hit our ship, so call TA87+3 to set \ bit 7 of the missile's byte #31, which marks the \ missile as being killed JSR EXNO3 \ Make the sound of the missile exploding LDA #250 \ Call OOPS to damage the ship by 250, which is a pretty JMP OOPS \ big hit, and return from the subroutine using a tail \ call .TA18 \ This is the entry point for missile tactics and is \ called from the main TACTICS routine below LDA ECMA \ If an E.C.M. is currently active (either our's or an BNE TA35 \ opponent's), jump to TA35 to destroy this missile LDA INWK+32 \ Fetch the AI flag from byte #32 and if bit 6 is set ASL A \ (i.e. missile is hostile), jump up to TA34 to check BMI TA34 \ whether the missile has hit us LSR A \ Otherwise shift A right again. We know bits 6 and 7 \ are now clear, so this leaves bits 0-5. Bits 1-5 \ contain the target's slot number, and bit 0 is cleared \ in FRMIS when a missile is launched, so A contains \ the slot number shifted left by 1 (i.e. doubled) so we \ can use it as an index for the two-byte address table \ at UNIV TAX \ Copy the address of the target ship's data block from LDA UNIV,X \ UNIV(X+1 X) to V(1 0) STA V LDA UNIV+1,X STA V+1 LDY #2 \ K3(2 1 0) = (x_sign x_hi x_lo) - x-coordinate of JSR TAS1 \ target ship LDY #5 \ K3(5 4 3) = (y_sign y_hi z_lo) - y-coordinate of JSR TAS1 \ target ship LDY #8 \ K3(8 7 6) = (z_sign z_hi z_lo) - z-coordinate of JSR TAS1 \ target ship \ So K3 now contains the vector from the target ship to \ the missile LDA K3+2 \ Set A = OR of all the sign and high bytes of the ORA K3+5 \ above, clearing bit 7 (i.e. ignore the signs) ORA K3+8 AND #%01111111 ORA K3+1 ORA K3+4 ORA K3+7 BNE TA64 \ If the result is non-zero, then the missile is some \ distance from the target, so jump down to TA64 see if \ the target activates its E.C.M. LDA INWK+32 \ Fetch the AI flag from byte #32 and if only bits 7 and CMP #%10000010 \ 1 are set (AI is enabled and the target is slot 1, the BEQ TA35 \ space station), jump to TA35 to destroy this missile, \ as the space station ain't kidding around LDY #31 \ Fetch byte #31 (the exploding flag) of the target ship LDA (V),Y \ into A BIT M32+1 \ M32 contains an LDY #32 instruction, so M32+1 contains \ 32, so this instruction tests A with %00100000, which \ checks bit 5 of A (the "already exploding?" bit) BNE TA35 \ If the target ship is already exploding, jump to TA35 \ to destroy this missile ORA #%10000000 \ Otherwise set bit 7 of the target's byte #31 to mark STA (V),Y \ the ship as having been killed, so it explodes .TA35 LDA INWK \ Set A = x_lo OR y_lo OR z_lo of the missile ORA INWK+3 ORA INWK+6 BNE TA87 \ If A is non-zero then the missile is not near our \ ship, so jump to TA87 to skip damaging our ship LDA #80 \ Otherwise the missile just got destroyed near us, so JSR OOPS \ call OOPS to damage the ship by 80, which is nowhere \ near as bad as the 250 damage from a missile slamming \ straight into us, but it's still pretty nasty .TA87 JSR EXNO2 \ Call EXNO2 to process the fact that we have killed a \ missile (so increase the kill tally, make an explosion \ sound and so on) ASL INWK+31 \ Set bit 7 of the missile's byte #31 flag to mark it as SEC \ having been killed, so it explodes ROR INWK+31 .TA1 RTS \ Return from the subroutine .TA64 \ If we get here then the missile has not reached the \ target JSR DORND \ Set A and X to random numbers CMP #16 \ If A >= 16 (94% chance), jump down to TA19 with the BCS TA19 \ vector from the target to the missile in K3 .M32 LDY #32 \ Fetch byte #32 for the target and shift bit 0 (E.C.M.) LDA (V),Y \ into the C flag LSR A BCC TA19 \ If the C flag is clear then the target does not have \ E.C.M. fitted, so jump down to TA19 with the vector \ from the target to the missile in K3 JMP ECBLB2 \ The target has E.C.M., so jump to ECBLB2 to set it \ off, returning from the subroutine using a tail call